Hydraulic fracturing in layered heterogeneous shale: The interaction between adjacent weak interfaces

Shale is known as the typical layered heterogeneous rock with multiple weak interfaces between different layers, and how to depict the propagation characteristic of fluid -driven fracture in such complex geological structures is critical for hydraulic fracturing engineering. Traditional investigations only focus on the intersecting behaviors of hydraulic fracture (HF) at a single weak interface. Herein, we reveal the propagation of HF in shale formation based on the extended finite element method (XFEM), where multiple weak interfaces and different rock properties are carefully considered in the sandwiched layered structure (SLS). The crack tip singularities at the weak interfaces due to the material discrepancy are characterized by the eigen-function expansion, and the unified enrichment function is constructed to evaluate the stress intensity factors (SIFs) directly without extra computations. The corresponding energy release rate (ERR) criteria are employed to determine the HF propagation direction. By the comprehensive analysis with various dip angles and material/structural properties, the propagation of HF moving across the layered heterogeneous shale is regarded as the interaction between adjacent weak interfaces. For the front weak interface, the inhibition effect could be seen in the fracture penetrating capacity by the rear interface, which reflects contrasting trends due to large material discrepancies of various weak interfaces. The propagation behavior at the rear weak interface is affected by the facilitation effect from the front interface, and the HF pre -offset at the front interface greatly enhances the facilitation effect on fracture penetrating through shale formations.